1. Field of the Invention
The present general inventive concept relates to an ink level detecting apparatus of an ink-jet printer, and more particularly, to an ink level detecting apparatus that utilizes a shrinkable ink pack having a height that varies according to an amount of remaining ink and a capacitor having an electric capacity that varies in response to the height variation of the shrinkable ink pack.
2. Description of the Related Art
In typical ink-jet printers, ink is supplied from an ink tank through an ink supplying passage to an ink ejection unit having an ink-jet head, and droplets of the ink are ejected through the ink-jet head. Therefore, ink level in the ink tank is lowered as printing progresses.
In the ink-jet printers, an amount of remaining ink (i.e., the ink level) in the ink tank should be measured somewhat precisely. In particular, when the ink level approaches a minimum level (that is, when the amount of remaining ink is insufficient), a user is not informed of this condition, and a next page cannot be printed for the following reasons.
First, if the ink is ejected until it is used up, the ink-jet head can be damaged. For example, according to one method of ejecting ink, a heating unit applies heat to the ink contained in an ink cell to eject the ink by rapidly expanding the ink. In this case, if the heating unit operates when the ink is not remaining in the ink cell, the heating unit or other parts around the heating unit can be damaged. Therefore, whether the ink reaches the minimum level should be reliably so as to prevent damaging of the ink-jet head.
Second, if the ink is used up before a current page is not yet completely printed, the current sheet of paper and the ink on the paper are wasted. Therefore, it must be detected whether the amount of ink for printing at least one page is remaining or not.
For these reasons, several ink level detecting apparatuses have been developed. For example, an ink level detecting apparatus that utilizes a capacitor having an electric capacity that varies depending on the ink level is described in U.S. Pat. No. 4,604,633 entitled “Ink-jet recording apparatus.” According to the ink-level detecting apparatus, an upper electrode and a lower electrode are respectively provided on an upper inner wall and a lower inner wall of an ink cartridge to face each other. An ink pack is interposed as an intermediate material between the two electrodes, thereby forming the capacitor. The ink pack shrinks as the ink therein is consumed, and the electric capacity of the capacitor varies in proportion to the shrinkage of the ink pack. Therefore, the ink level can be detected by measuring the variation of the electric capacity.
However, according to the ink-level detecting apparatus described above, a distance between the two electrodes (i.e., the upper and lower electrodes) is too large to obtain a signal having a sufficient intensity. Further, it is assumed that an area of the ink pack formed of a flexible film decreases approximately in proportion to the amount of ink used. However, not only the area of the ink pack but also the height of the ink pack changes according to the use of the ink. That is, the distance between the upper electrode and the lower electrode changes according to the use of the ink. Since the electric capacity between the two electrodes is affected by the distance between the two electrodes more than the area of overlap between the two electrodes, an irregular height variation of the ink pack affects the electric capacity significantly. Therefore, there is a problem in that it is difficult to implement the apparatus described above, and even if the apparatus is implemented, the ink level cannot be detected reliably.